1. Field of the Invention
The present invention relates generally to a dot-matrix printer capable of printing characters, such as letters, symbols and graphical representations, with a matrix of dots. More particularly, the invention is concerned with such a dot matrix printer which is free from printing troubles due to a drop in drive voltage of a print head, or an abnormal rise in temperature of the print head, when dots are formed continuously with a relatively high density.
2. Description of Related Art
In a common dot-matrix printer, print elements provided on a print head are disposed in a row and selectively activated to form dots in a vertical row of a dot matrix pattern corresponding to a character.
When dots are printed continuously with a high density, the temperature of the print head rises excessively. As a result, the excitation coils of the print head drive circuit are broken due to the excessive rise in the temperature. In addition, soldered joint portions melt so that joints separate.
In the art of dot-matrix printing, it is also recognized that excessive temperature rise of a print head results in poor printing quality. More specifically, in the case of a wire-dot printer, the insulating characteristic of insulators for solenoids that activate the print wires is reduced when the temperature of the print head is excessively elevated. Consequently, the solenoids may fail to be energized as required, whereby the corresponding dots may not be formed. Further, an excessive rise of the print head temperature causes the thermal ribbon to be fused by portions of the print head other than the activated heat-generating elements. In this case, a print sheet or other recording medium is soiled with ink or the characters may not be printed clearly.
A control arrangement for a dot-matrix printer to solve the above-mentioned problem is proposed in Japanese Unexamined patent application No. 59-234732, filed in 1984, which was laid open to public inspection on May 30, 1986 under Provisional Publication No. 61-112649 and corresponds to U.S. patent application Ser. No. 779,095 filed Sep. 23, 1985, now U.S. Pat. No. 4,653,940. These applications were filed by the same assignee who files the present application.
In the dot-matrix printer constructed as described above, the number of dots which are formed by the activated print elements is added to the current content of the dot-number memory in order to sum up the amount of heat generated by the print head. To better define the heat generated, a predetermined number is subtracted from the current content of the dot-number memory in order to compensate for the amount of heat (which has been generated in the print head) which is dissipated in each lapse of the predetermined time interval. In other words, the predetermined number to be subtracted is equivalent to the sum of heat dissipated in the predetermined time interval. Accordingly, the current content of the dot-number memory represents the current amount of heat accumulated in the print head. For this reason, the temperature of the print head or the amount of heat accumulated in the print head may be estimated by judging whether the current content is greater than a reference value. If it is judged that the current content of the dot-number memory has exceeded the predetermined reference value, a special printing mode is executed to cool the print head.
However, as a result of the continuous study made by the assignee of the present application, it is known that above mentioned printer has a problem. That is, the reference value selected is so small that the temperature of the print head does not rise, in any case, higher than an allowed temperature when the judging means judges the current content of the dot-number memory has reached the reference value.
Therefore, the special printing mode is executed before the temperature of the print head has increased appreciably. When printing data comprising many printing dots are transmitted from the host computer shortly after the power has been supplied, the current content of the dot-number memory exceeds the reference value and the special printing mode is executed even through the temperature of the print head is still low. Thus, the printing speed is decreased below the normal value and the efficiency of the printing operation is reduced.
An object of the present invention is to provide a dot-matrix printer which is able to form continuously high quality letters, symbols and graphical representations without the identified printing problems normally associated with an excessive rise in the temperature of the print head. This is done by counting the number of dots to be printed and keeping printing speed normal even when the counted number of dots are a large value if the temperature of the print head has not yet risen or has risen by a minor amount.
According to the present invention, there is provided a dot-matrix printer including a print head having plural print elements which are selectively activated, during movement of the print head, to form a dot or dots in a vertical row of a dot matrix pattern representing a character, comprising: a dot-number memory; adding means for adding to a current content of the dot-number memory the number of dots which are formed by the print elements during movement of the print head; time-measuring means for measuring a time lapse; subtracting means for subtracting a predetermined value from the current content of the dot-number memory each time the time-measuring means has measured a predetermined time interval; a temperature detecting means for detecting a temperature of a print head; a subtraction value changing means for changing a subtraction-value to be subtracted from the current content of the dot-number memory according to the temperature detected by the temperature detecting means; judging means for checking whether the current content of the dot-number memory has exceeded a predetermined reference value; and printing control means for controlling the print head so as to cool the print head if the judging means has judged that the current content of the dot-number memory has exceeded the predetermined reference value.
In such a manner, when the current content of the dot-number memory exceeds the predetermined reference value a special printing mode is executed to cool the print head. As a result, the dot-matrix printer of this invention can print characters such as letters, symbols and graphical representations of high quality and yet be freed from the printing problems caused by an excessive rise in the temperature of the print head. This is accomplished by using a subtraction value changing means to change the subtraction value to a preferable value according to the temperature detected by a temperature detecting means. Thus, the value in the dot-number memory is kept at the preferable value when the temperature of the print head does not rise excessively. Consequently, the special print mode is not executed when the temperature of the print head rises normally. Accordingly, printing speed is kept at the standard speed when the temperature of the print head is in a normal range.